Apparatus for eliminating destructive, self-excited vibrations in steam generators



2, 1966 J. J. COHAN ETAL 3,263,654

APPARATUS FOR ELIMINATING DESTRUGTIVE. SELF-EXCITED VIBRATIONS IN STEAM GENERATORS Filed June l7, 1964 4 Sheets-Sheet l INVENTORS LEO J, J. COHAN 34 WILLIAM J. DEANE FIG. I Y Magda ATTORNEY 2, 1966 L J. J. COHAN ETAL 3,263,654

APPARATUS FOR EL IMINATING DESTRUCTIVE. SELF-'EXCITED VIBRATIONS IN STEAM GENERATORS Filed June 17, 1964 4 Sheets-Sheet 5 GAS FLOW FIG. 3

1/2 WAVE DISPLACEMENT MODE 151 harmonic FULL WAVE DISPLACEMENT MODE 2nd harmonic 1 1/2 WAVES 3 rd harmonic 2 WAVES 4 'th harmonic 0 2O 4O 6O 80 I00 DUCT WIDTH, FIG. 4

IN VENTORS LEO d. J. COHAN WlLLlAMl J. DEANE ATTORNEY Aug. 2, 1966 L. J. J. COHAN ETAL APPARATUS FOR ELIMINATING DESTRUCTIVE. SELF-EXCITED Filed June 17, 1964 VIBRATIONS IN STEAM GENERATORS 4 Sheets-Sheet 4.

CPS

NATURAL FREQUENCY FIG.

CONVECTION PASS WIDTH, FT

SONIC VIBRATION BAFFLE PLACEMENT FOR OIL AND GAS FIRED UNITS TOTAL DUCT NUMBER OF BAFFLE LOCATION IN WIDTH FT BAFFLES PERCENT OF TOTAL DUCT WIDTH (LEFT) I RIGHT) 0 TO 22 NONE I00 22 TO 29 I 25 75 29 TO 39 2 25 I8.75 56.25

39 TO 52 3 25 I8.75 I4.06 42.I9

52 T0 69 4 25 I8.75 I4.06 IO.55 3|.64

69 TO 87 5 25 I8.75 I4.0 7.9I 23.73

FIG. 6

INVENTORXI LEO J. J. COHAN WILLIAM J. DEANE ATTORNEY United States Patent 3,263,654 APPARATUS FOR ELIMINATING DESTRUCTIVE,

SELF-EXCITED VIBRATIONS IN STEAM GEN- ERATORS Leo J. J. Cohan, Simsbury, and William J. Deane, Hartford, Conn., assignors to Combustion Engineering, Inc., Windsor, Conn., a corporation of Delaware Filed June 17, 1964, Ser. No. 375,713 8 Claims. (Cl. 122-4) This invention relates to apparatus for eliminating self-excited vibration problems created by gases flowing through a duct, and over. tubular members positioned within the duct and extending in a direction transversely of the gas flow path, and in particular to apparatus for eliminating such self-excited vibration problems existing in the rear gas pass of a steam generator.

In recent years, steam generators are becoming increasingly larger. As the size of these units have increased, the problem of self-excited vibrations has become more prevalent. The vibrations are caused by the combustion gases in flowing over tubular panels positioned in the rear gas pass of the steam generator. Whenever standing wave forms are created in the duct or chamber which are in resonance or harmony with the exciting frequency, vibrations are created which can lead to the ultimate failure of equipment, and also creates noise which is highly disturbing to operating personnel.

To overcome this problem, bafiles have been positioned within the rear gas pass of units where vibrations exist. These bafiies break up the standing wave forms, thus eliminating or substantially reducing the vibrations. However, in the past there have been no means by which it can be forecast which units, when constructed, will encounter these problems, or if they are encountered, just where the baffles should be located to most economically and efficiently eliminate the problem. Thus it is necessary to stop operation of the unit at least once, or in some instances a number of times, to install the baffles in the gas pass. This can be extremely expensive and troublesome to utility companies where the steam generator is being used in conjunction with a turbine for generating electricity.

It is the object of our invention to provide for a systematic manner of determining where and how many baffles should be positioned within a gas pass of a steam generator to economically and efiiciently eliminate selfexcited resonant vibrations therefrom.

The invention will be more readily understood from the following description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a diagrammatic representation of a vertical section of a steam generator;

FIG. 2 is an enlarged sectional view of a portion of the rear gas pass of a steam generator, taken on line 2-2 of FIG. 1;

FIG. 3 is an illustration of typical gas pulsations existing in the gass pass of a steam generator;

FIG. 4 is an illustration of various standing wave forms which may exist in a duct;

FIG. 5 is a graph showing what standing wave forms which may exist in a gas pass of a steam generator based on the width of the gas pass and the frequency of the vibrations; and,

FIG. 6 is a chart which illustrates the number and location of baffles determined in accordance with our invention in order to preclude the presence of damaging self-excited vibrations therefrom.

Looking now to FIG. 1, the steam generator illustrated contains a furnace 20, the walls of which are lined with steam generating tubes 22. Burners 24 supply the furnace with fuel and air which is burned therein. The hot 'combustion gases travel upwardly through the furnace and out through the horizontal gas pass 26, then continue downwardly through vertical gas pass 28 before being exhausted.

The combustion gases give up heat to the heat exchange surfaces contained within the boiler, thus creating superheated steam to be used in driving a turbine for generating electricity. Water enters economizer 30 and is heated to some extent before flowing to the steam and water drum 32. Water flows from drum 32 downwardly to inlet header 34, which distributes the water to the tubes 22. A steam and water mixture leaves tubes 22, flowing into outlet header 36, from where it is conveyed back to drum 32. The steam separates out in drum 32, flowing through lines 38 to the primary superheater sections 39, 40 and 41 which are in series flow relationship. S-uperheated steam leaving section 41 flows by way of pipe 42 to secondary superheater section 44. The steam is superheated to a greater extent in flowing through secondary superheaters 44, 46 and 48%. The superheated steam flows from section 48 to its ultimate point of use, for example a steam turbine used to generate electricity.

It is in the rear gas pass 28 where the primary superheater sections 39, 40 and 41 are located where vibration problems are usually encountered, and. in accordance with our invention baflles 50 are positioned in this area, thus forming a plurality of parallel passageways through which the combustion gases pass.

When a gas flows transversely over a tube, the wake behind the tube is no longer regular or streamlined, but exhibits distinct clockwise and counterclockwise vortices in a regular, periodicmanner, as illustrated in FIG. 3. This shedding, from alternate sides of the tubes, causes a varying force on the tubes perpendicular to the normal flow of the gas. When the vortex shedding frequency coincides with the natural frequency of the gas column, vibration occurs. This is synonymous to motivating a large pendulum with a small force applied with a hammer at the natural frequency of the pendulum. If this small force is applied during each cycle, the amplitude will continually increase. This is a condition of resonance between the exciting force and the natural harmonic motion of the pendulum. In a gas pass containing tubes therein, the vortices are the hammer, and the gas stream is the pendulum.

In any duct containing tubular objects where a gas is passing therethrough, if the exciting frequency is in resonance with the frequency of the chamber or duct, standing wave forms are created. Whether or not the existing frequency is in resonance with the frequency of the chamber or duct is dependent on such factors as gas velocity, pressure, temperature, size and vertical and horizontal spacing of the tubular objects, width of the duct or chamber, and others.

FIG. 4 depicts some typical wave formations which may exist in a duct. As can be seen, a one-half wave, or first harmonic, would extend across the entire width of the duct. Likewise, a full wave, or second harmonic, extends across 50 percent of the duct width; a 1 /2, or third harmonic, would extend across 33 /3 percent of the duct width, etc. The nodes are the intersection points of the loops of the wave formations, and to eliminate any standing wave formations from a duct, the waves should be broken or interrupted at a point between two nodes, preferably, although not necessarily, at their midpoint. Thus one manner of deresonating a resonant duct or chamber is to subdivide that chamber by placing baflies therein, thus forming a plurality of chambers which would have a frequency that differed from, and was no integral sub-multiple of, the exciting frequency.

From calculated performance conditions, the graph shown in FIG. was plotted of the resonant frequency of a duct and the vortex shedding frequency (based on a gas temperature of 1460 F. absolute). By taking measurewould be 22 feet or greater, and this would be undesirable. Thus our means of determining the placement of baffles should not be applied to steam generating units which have a rear gas pass in excess of 87 feet.

rnents in steam generating units in existence, it was de- 5 FIG. 6 is a chart showing the number and location of termined that the natural frequency in the rear gas pass bafiles that should be placed in accordance with our inof most units was in the range of 40100 cycles per secvention to overcome vibration problems, for units having ond (c.p.s.). By checking the graph of FIG. 5, the hara gas pass width falling within the range of 22-87 feet.

monic that might exist in any unit of a given width can The baffle placement series developed provides cavities be fairly accurately determined. It was also found that 10 at a alw ys in a 4 0 3 Width ratio from the left Side, vibration trouble rarely occurred in units where the rear except the last two cavities which are in a 3 to 1 width gas was less than 22 feet in width. ratio. This can be seen in FIG. 6. The only way that From FIG. 4 it can be seen that a baflie placed anythe far right bafiie could be at a nodal point would be if Where would theoretically eliminate a standing first harthere Were tWO Waves, Which ran through the battle This monic or one-half wave, with the mid oi t, or dead would mean three loops on one side of the baffle and one center, being the optimum location. The midpoint, howloop on the other side. Since there will be no cavities ever, would be the worst possible location to deresonate Over 22 t Wide in accordance With Our irlVeIitierl, 11/2 a second harmonic, or full wave, since there is a displace- Waves Would be required to eXiSt at a frequency Well in ment node at the center. Thus a baffle placed here would eXeeSS Of 100 -P- Although this eehdibe useless, since the wave could travel through it; or a tion is P s it is highly improbable based on a Study first harmonic or one-half wave would exist on each side. of units in eXiStehee, Where the frequency 0t most hits The optimum bathe location for a full wave would be the fell Within the r arlge 0f 40-100 -P' one-quarter point, furthest from any of the nodes. But 2 Shows the battle Placement in the rear gas P both one-quarter points should not be bafi led, because this Of a unit in accordance With the 6 of a urlit could produce a one-half wave in the middle. Thus it can 25 the gas P Width of which falls Within the range of be seen that to develop a fixed formula for baffle place- 69-37 feet- As Shown, the baffles 50 are Secured in P ment which can be applied to any boiler before it is contieh by rods 52 Welded to tubes 40 0f the SllPerheater, structed (and thus before it is known what harmonics although y suitable manner of Securely Positioning and might exist in the rear gas pass) presents difficulties. Supporting the bellies can be useti- The bellies Should be Based on the measurements taken of steam generating made a Suitable metal capable of withstanding the units in existence, a systematic manner was derived for temperatures of the Combustion gases (800-15000 determining the most economic and efiicient number and Although the invention has been illustrated in eerlrieeposition of bafi les to be placed in the rear gas pass to elimtieh With Primary superheater Surfaces, it Vibration P inate vibration problems. Our formula is applicable for lems are encountered in the Secondary euperheater, all units Whose rear gas pass width falls within the range heater, or eceherhiler areas, battles Could be Placed in of 22-87 feet, Abo 22 f t, h primary eoncem i the these areas also in accordance with our invention. Most full wave, so a bathe i placed t th One-quarter point of the vibration problems which have occurred in the past The one-quarter point will also eliminate a one-half and have been in the Primary of low 1temperature StlPerheater 1 /2 standing wave (see FIG. 4). area. It has also been found that most vibration problems This single baflie placement is adequate until the larger occur on Oil or g fired Steam generatorsof the two cavities approaches 22 f or :2, 222 f t As illustrated in FIG. 2, the bafiies are of a height so as where W i the width f th gas pass Th when w the to extend from the top of the tubular panels 41 to the width of a unit, is greater than 29 feet, more than one bottom of the tubular Panels It y be Possible to bathe i needed, If we quarter point h l eavity' eliminate vibration problems by using three smaller with a second bafi'le, there will then be three cavities 45 baffles, vertically Spaced, in P121Ce of each bathe Shown in fo ed in th f ll i proportion; FIG. 2. Each of these three baffles would extend from AW; A(%)W;(%)(%)W; the top of one panel to a point just slightly below the or 25% W: 18.75% W:56.25% W. bottom of the panel.

Again the largest of the three cavities may grow until The figure 22 feet has more or less been arbitrarily 56.25% W=22 feet; or until the width of a unit is 39 feet. chosen as a Starting Point for developing the bathe P Thus on units whose gas pass width is greater than 39 feet, formula, baed on P experience It Could be three baflies would be positioned, forming four cavities as Possible to start Wlth a figure Within the range of 2025 f ll feet, for example, and still apply the formula with con- W: (34 )W: 2 )sW; siderable success in preventing vibration problems. or 25%W:18.75%W:14.06%W:42.19%W. What We e i Thus it can be Seen that the number and position 0f 1. In combination, afurnace wherein hot combustion bafiies are determined so as to leave cavities in accordance gases are generatefl by the burning of fuel therein with the following. tical gas pass having from, rear and side walls connected 1414/92, 32 1 2 to the furnace through which the combustion gases flow,

:14 (3A )n 1W: (3A )nW 50 a plurality of tube panels positloned within the gase pass, wherein W is the total width of the gas pass, and n is the each tubfi panel belngmade up of a p-lurahty 9 parallel vert1cally spaced horizontal tube portions WhlCh extend number of bafiies. When the last cavity in the series, or in a direction runnin fro th f H h u W reaches 22 feet another baffle is needed. g R1 6 0t 6 rear wa and f d t vertical baflie means positioned within the gas pass WhlCh 1 manner 0 e ermining the numbenand posltlon are parallel 'to the tube panels, the number and position of bafiles can be used on units havlng a width up to as of the baffle means being determined substantially in aclarge as 87 feet. Above 87 feet, the first cavity, or AW, cordance with the following:

Total duct Number of (Side 1) (Side 2) width, ft. bafiies Bafiie location in terms of percent of total duet width 100 1 25 2 25 18.75 56. 25 3 25 1s. 75 14. 05 42.19 4 25 18.75 14. 06 10. 55 .64 5 25 18.75 14.06 10.55 i 7 91 J 23 7s to thereby preclude the presence of damaging self-excited vibrations caused by the combustion gases flowing past the tube panels.

2. In combination, a furnace wherein hot combustion gases are generated by the burning of fuel therein, a vertical gas pass having front, rear and side walls connected to the furnace through which the combustion gases flow, a plurality of tube panels positioned within the gas pass, each tube panel being made up of a plurality of parallel, vertically spaced horizontal tube portions which extend in a direction running from the front to the rear wall, and vertical baflle means positioned within the gas pass which are parallel to the tube panels, the number and position of the baflle means being such that a plurality of parallel passageways result, the relationship of the successive widths of the passageways defined by the following:

wherein W is the total width of the gas pass and W is within the range of 22-87 feet, n is the number of baifle means to thereby preclude the presence of damaging selfexcited vibrations caused by the combustion gases flowing past the tube panels and W, the largest passageway, approaches or equals 22 feet but does not exceed 22 feet.

3. In combination, a furnace wherein hot combustion gases are generated by the burning of fuel therein, a vertical gas pass having four enclosing walls connected to the furnace through which the combustion gases flow, a plurality of tube panels positioned within the gas pass, each tube panel containing a plurality of parallel, vertically spaced horizontal tube portions which extend in a direction parallel to two of the enclosing walls of the gas pass, and vertical baffle means positioned within the gas pass which are parallel to the tube panels, the number and position of the baffle means being such that a plurality of parallel passageways result, the relationship of the successive widths and number of the passageways defined by the following:

wherein W is the total Width of the gas pass and W is within the range of 22-87 feet, n is the number of baffle means, and W22 feet, (%)W being the largest passageway, to thereby preclude the presence of damaging self-excited vibrations caused by the combustion gases flowing past the tube panels.

4. In combination, a furnace wherein hot combustion gases are generated by the burning of fuel therein, a vertical gas pass means having four enclosing walls connected to the furnace through which the combustion gases flow, a plurality of tube panels positioned within the gas pass means, each tube panel containing a plurality of parallel, vertically spaced horizontal tube portions which extend in a direction parallel to two of the enclosing walls of the gas pass, and a vertical baflle means positioned within the gas pass which is parallel to the tube panels, said two of the enclosing walls of the gas pass being at least 22 feet but less than 29 feet apart, with the baflle means being positioned approximately 25 percent of the total width distance from one of said enclosing walls, to thereby preclude the presence of damaging self-excited vibrations caused by the combustion gases flowing past the tube panels.

5. In combination, a furnace wherein hot combustion gases are generated by the burning of fuel therein, a vertical gas pass means having four enclosing walls connected to the furnace through which the combustion gases flow, a plurality of tube panels positioned within the gas pass means, each tube panel containing a plurality of parallel, vertically spaced horizontal tube portions which extend in a direction parallel to two of the enclosing walls of the gas pass, and two vertical baffle means positioned within the gas pass which are parallel to the tube panels, said two of the enclosing walls of the gas pass being at least 29 feet but less than 39 feet apart, with the two baffle means being positioned approximately 25 percent and 43.75 percent of the total width distance from one of said enclosing walls, respectively, to thereby preclude the presence of damaging self-excited vibrations caused by the combustion gases flowing past the tube panels.

6. In combination, a furnace wherein hot combustion gases are generated by the burning of fuel therein, a vertical gas pass means having four enclosing walls connected to the furnace through which the combustion gases flow, a plurality of tube panels positioned within the gas pass means, each tube panel containing a plurality of parallel, vertically spaced horizontal tube portions which extend in a direction parallel to two of the enclosing walls of the gas pass, and three vertical baffle means positioned within the gas pass which are parallel to the tube panels, said two of the enclosing walls of the gas pass being at least 39 feet but less than 52 feet apart, with the three baffle means being positioned approximately 2.5 percent, 43.75 percent, and 57.81 percent of the total width distance from one of said enclosing walls, respectively, to thereby preclude the presence of damaging se-lfexcited vibrations caused by the combustion gases flowing past the tube panels.

7. In combination, a furnace wherein hot combustion gases are generated by the burning of fuel therein, a vertical gas pass means having four enclosing walls connected to the furnace through which the combustion gases flow, a plurality of tube panels positioned within the gas pass means, each tube panel containing a plurality of parallel, vertically spaced horizontal tube portions which extend in a direction parallel to two of the enclosing walls of the gas pass, and four vertical baflle means positioned within the gas pass which are parallel to the tube panels, said two of the enclosing walls of the gas pass being at least 52 feet but less than 69 feet apart, with the four baffle means being positioned approximately 25 percent, 43.75 percent, 57.81 percent, and 68.36 percent of the total width distance from one of said enclosing walls, respectively, to thereby preclude the presence of damaging selfexcited vibrations caused by the combustion gases flowing past the tube panels.

8. In combination, a furnace wherein hot combustion gases are generated by the burning of fuel therein, a vertical gas pass means having four enclosing walls connected to the furnace through which the combustion gases flow, a plurality of tube panels positioned within the gas pass means, each tube panel containing a plurality of parallel, vertically spaced horizontal tube portions which extend in a direction parallel to two of the enclosing walls of the gas pass, and live vertical baffle means positioned within the gas pass which are parallel to the tube panels, said two of the enclosing walls of the gas pass being at least 69 feet but less than 87 feet apart, with the five baflle means being positioned approximately 25 percent, 43.75 percent, 57.81 percent, 68.36 percent, and 76.27 percent of the total width distance from one of said enclosing walls, respectively, to thereby preclude the presence of damaging self-excited vibrations caused by the combustion gases flowing past the tube panels.

References Cited by the Examiner UNITED STATES PATENTS 2,893,509 7/1959 Baird l-69X KENNETH W. SPRA-GUE, Primary Examiner. 

1. IN COMBINATION, A FURNACE WHEREIN HOT COMBUSTION GASES ARE GENERATED BY THE BURNING OF FUEL THEREIN, A VERTICAL GAS PASS HAVING FRONT, REAR AND SIDE WALLS CONNECTED TO THE FURNACE THROUGH WHICH THE COMBUSTION GASES FLOW, A PLURALITY OF TUBE PANELS POSITIONED WITHIN THE GASE PASS, EACH TUBE PANEL BEING MADE UP OF A PLURALITY OF PARALLEL VERTICALLY SPACED HORIZONTAL TUBE PORTIONS WHICH EXTEND IN A DIRECTION RUNNING FROM THE FRONT TO THE REAR WALL, AND VERTICAL BAFFLE MEANS POSITIONED WITHIN THE GAS PASS WHICH ARE PARALLEL TO THE TUBE PANELS, THE NUMBER AND POSITION OF THE BAFFLE MEANS BEING DETERMINED SUBSTANTIALLY IN ACCORDANCE WITH THE FOLLOWING: 